This invention relates to a system for treating particulate ladened boiler flue gas with a conditioning agent to improve the removal of particulate matter by electrostatic means and, more particularly, relates to an SO.sub.3 flue gas-conditioning system which operates automatically to provide an effective flow of SO.sub.3 conditioning agent and to control the catalytic converter temperature while meeting a system demand for SO.sub.3.
The increasing demand for electrical power has forced electrical utilities to burn increasing quantities of fossil fuels. However, electric utilities face increasing environmental standards that are imposed upon their operation. In trying to satisfy the divergent demands of increasing power and decreased air pollution, electrical utilities have turned to burning low-sulfur coals, i.e., coals having less than one percent sulfur content, to fire their boilers and generate the steam needed for electrical power generation.
Electrostatic means such as electrostatic precipitators have long been used by electrical utilities to remove particulate matter such as fly ash from boiler flue gas. The efficiency of operation of the electrostatic precipitators in the removal of particulate matter from boiler flue gas is dependent, in part, upon the electrical resistivity of the entrained particulate matter in the boiler flue gas. The entrained particulate matter in the boiler flue gas expelled from a boiler fired with low-sulfur coal has been found to have a high resistivity, for example, 10.sup.13 ohm-cm resistance and more. It has also been determined that the efficient removal of particulate matter by electrostatic precipitation occurs when itsresistivity is on the order of about 10.sup.8 ohm-cm and that when the resistivity of the particulate matter is higher, for example, on the order of 10.sup.13 ohm-cm, the efficiency of electrostatic precipitation is substantially reduced. Thus, reduced efficiency in the operation of electrostatic precipitators with the flue gas from low-sulfur coals has been attributed to the higher resistivity of such flue gas particles. Any reduction of the ability of an electrostatic precipitator to remove particles from the flue gas can offset, of course, the reduced or potentially reduced air pollution sought through the use of the more expensive low-sulfur coals.
The efficiency of an electrostatic precipitator in the removal of particulate matter from boiler flue gas can be increased by introducing a conditioning agent in the boiler flue gas prior to its entrance into the electrostatic precipitator to reduce the resistivity of the entrained particles within the boiler flue gas. Various chemicals, such as water, anhydrous ammonia and various ammonia-bearing solutions, sulfuric acid, sulfur trioxide and phosphoric acid, have been used as conditioning agents for boiler flue gas.
U.S. Pat. No. 3,284,990 discloses a method of improving the electrostatic precipitation of particles by adding phosphorous pentoxide to the particles prior to their electrostatic precipitation.
U.S. Pat. No. 3,523,407 discloses a method of improving the electrostatic precipitation of particles from a flue gas by adding preselected amounts of ammonia and water to the flue gas.
U.S. Pat. No. 3,665,676 discloses a system to condition the particles of boiler flue gas by the use of a salt solution such as a solution of ammonium sulfate or ammonium bisulfate. The salt solution is injected into the flue gas prior to entering the electrostatic precipitator and the system includes a metering means for controlling the amount of conditioner injected into the flue gas. U.S. Pat. No. 3,665,676 indicates that, if desired, conventional automatic controls can be provided to open the metering means when the flue gas reaches the desired operating temperature or to close it should the temperature fall below operating temperature. In addition, automatic controls can also be made to open the metering means to provide the amount of conditioner needed in proportion to the volume of gas to be conditioned.
U.S. Pat. No. 3,689,213 discloses a process for treating flue gas in which gaseous sulfur trioxide is generated in the immediate vicinity of the point of use as required by the quantity of fossil fuel being burned per unit time and is then introduced into the flue gas at a predetermined rate to facilitate fly ash removal by an electrostatic precipitator. In the system of U.S. Pat. No. 3,689,213, air and gaseous sulfur dioxide are heated in a heat exchanger to a temperature required for oxidation of sulfur dioxide to sulfur trioxide. The air and sulfur dioxide are passed through a catalytic converter for conversion of the sulfur dioxide to sulfur trioxide prior to its injection into the boiler flue gas.
U.S. Pat. No. 3,772,178 discloses a system for the production of sulfur trioxide for flue gas conditioning including means to deliver a source of sulfur such as sulfuric acid to a vaporizer in proportion to the amount of flue gas from the boiler measured in terms of the electrical output generated at a particular time. As the production of flue gas changes in the boiler system, the proper ratio of acid to flue gas is automatically maintained by a control responsive to a signal coming from a boiler capacity index gauge to control the volume of sulfur trioxide being produced. The vaporizer is provided with a mixture of fresh air and a combustion gas from a natural gas or oil to convert the sulfuric acid to sulfur trioxide. The amount of combustion gas directed into the combustion chamber is automatically controlled by the temperature of the sulfur trioxide as indicated by temperature controllers mounted at the top and bottom of the vaporizer in the path of the output gas. The temperature controllers maintain the temperature of the vaporizer in the range for efficient production of sulfur trioxide. An additional temperature controller at the exit of the vaporizer turns off the burner when the temperature at the exit exceeds 1200.degree. F. (649.degree. C.).
Developments in flue gas conditioning systems have more recently focused on sulfur trioxide (SO.sub.3) as a flue gas-conditioning material. These flue gas-conditioning systems have included systems which store liquified sulfur which is supplied to a sulfur burner in which the sulfur is converted by combustion predominantly to sulfur dioxide. The systems then pass the sulfur dioxide to a catalytic converter which employs a vanadium pentoxide catalyst to convert the sulfur dioxide into sulfur trioxide. The sulfur trioxide created by such systems is piped to a nozzle system for injection into ducts carrying the boiler flue gas and its entrained particulate material to reduce the electrical resistivity of the flue gas particulate matter for removal by an electrostatic precipitator.
Several prior systems have been disclosed to control such SO.sub.3 flue gas-conditioning systems. Such a system is disclosed, for example, in U.S. Pat. No. 3,993,429. In the system of U.S. Pat. No. 3,993,429 and in commercial systems resulting from this patent, a flow of heated air is forced into the sulfur burner; the temperature of the gas leaving the sulfur burner is sensed; and the sensed output temperature of the sulfur burner is used to control either the temperature of a flow of air forced into the sulfur burner, or the portion of a flow of heated air that is forced into the sulfur burner. The system of U.S. Pat. No. 3,993,429 increases or decreases the temperature of the air directed into the sulfur burner, or the portion of the heated air directed into the sulfur burner, in the event the burner outlet temperature is too low or too high, respectively. The system of U.S. Pat. No. 3, 993,429 thus attempts to regulate the operating temperature of the sulfur burner and the catalytic converter downstream of the sulfur burner by regulating an air heater or an air flow diverter valve, or both, upstream of the sulfur burner.
U.S. Pat. No. 3,993,429 also discloses a system in which the temperature of operation of the catalytic converter is controlled by providing a second flow of air to be mixed with the output of the sulfur burner, detecting the temperature of the mixture of the second flow of air and the gases leaving the sulfur burner and varying the temperature of the air in the second flow of air to maintain a desired operating temperature for the catalytic converter. U.S. Pat. No. 3,993,429 further discloses that SO.sub.3 flue gas conditioning systems can operate by sensing the rate of coal combustion and varying the rate of flow of sulfur into a sulfur burner in response to the rate of coal combustion.
U.S. Pat. No. 4,770,674 discloses a system for conditioning flue gas for an electrostatic precipitator, including equipment for converting sulfur into sulfur trioxide. The disclosed systems of U.S. Pat. No. 4,770,674 include a sulfur burner to produce oxidized sulfur, a catalytic converter to convert the oxidized sulfur to sulfur trioxide, and means to control sulfur and air inputs to the sulfur burner. Various inputs to the control means are disclosed, including the outlet temperature of the catalytic converter and such operating parameters of the exhaust stage of the system as the output temperature of the exhaust gas from the precipitator, the flow rate of the exhaust gas, the power delivered to or the speed of, an induced draft fan, if any, the opacity of the exhaust gas within the stack, and the power dissipated by the precipitator.
U.S. Pat. No. 4,779,207 discloses a system for preconditioning flue gas for electrostatic precipitation. The system of U.S. Pat. No. 4,779,207 includes a source of an SO.sub.3 conditioning agent, a means for controllably adding the conditioning agent to the flue gas, a means for detecting the input power level of the electrostatic precipitators and control means for monitoring the input power level and controlling the amount of conditioning agent added to the gas to substantially maintain input power to the electrostatic precipitator to predetermined levels.
A controller commercially available from Castlet (Electronic Engineers) Ltd., of 14 Crofton Drive, Lincoln, England, can control an electrostatic precipitator by detecting the presence of deleterious back ionization and intermittently applying voltage to the charging electrodes of the precipitator to minimize back ionization. The Castlet controller detects back ionization by interrupting the applied charging voltage at its peak value and comparing, after a preset time, the actual charging electrode voltage with a programmed charging electrode voltage to identify excess charging electrode decay rate indicative of back ionization. The Castlet controller uses the difference in actual and programmed charging electrode voltage to determine a rate of application of voltage to the charging electrodes in an effort to optimize precipitator operation in the presence of conditions of back ionization.
U.S. Pat. No. 5,032,154 discloses, among other things, a system which provides direct, automatic control of the opacity of the effluent of a coal-fired boiler to maintain minimal opacity of the flue gas effluent passing from the boiler into the atmosphere. Systems of U.S. Pat. No. 5,032,154 provide a controlled flow of an agent, such as sulfur trioxide, to the boiler flue gas to condition its particulate matter for removal by electrostatic means, monitor precipitator power, and the opacity of the boiler flue gas after it leaves the electrostatic particle-removal means, and vary the controlled flow of conditioning agent to hunt and operate at conditioning agent flow rates determined from flue gas opacity alone or combined with precipitator power.
Other conditioning systems are shown, for example, in U.S. Pat. Nos. 3,686,825; 4,042,348; 4,284,417; 4,466,815; 4,533,364; and 4,624,685.